1. Field of the Invention
The present invention relates to a porous insulating material and its laminates, that exhibit excellent electrical properties (low dielectric constant) and heat resistance, and particularly it relates to a porous insulating material and its laminates that are resistant to dielectric deterioration by dielectric breakdown or the like and are strongly adhesive or adhesive, and that are highly useful for high-frequency electronic parts, and especially distribution substrates.
Throughout the present specification, “insulating film” will refer to an electrical insulating film. “Fine continuous pores” will refer to fine pores that form continuous channels from one side to another, where the fine pores preferably run in a nonlinear fashion from one side to the other on a nonlinear path.
2. Description of the Related Art
With the rapid increase in communication data in recent years there has been an increasing desire for miniaturization, weight reduction and higher speeds for communication devices, which has led to a demand for suitable low-dielectric constant electrical insulating materials. In particular, the frequency band ranges of the waves used for portable mobile communications such as automobile phones, digital cellular phones and satellite communications are the high-frequency bands in the MHz and GHz ranges. In the rapid development of communication devices used as the means for such communication, designs have been produced for mounting small high-density boards, substrates and electronic elements. For miniaturization and weight reduction of communication devices suited for high-frequency ranges such as the MHz to GHz bands, it is necessary to develop electric insulating materials having both excellent high-frequency propagation characteristics and appropriate low dielectric constant characteristics.
Specifically, energy loss occurs in circuit elements during the course of propagation, and is known as dielectric loss. This energy loss is undesirable because it is expended as heat energy in the circuit element and is released as heat. The energy loss is produced in low frequency ranges by variation in the electric field of the dipole created by dielectric polarization, and is produced in high frequency ranges by ionic polarization and electronic polarization. The ratio of the energy expended in a dielectric material and the energy stored in the dielectric material in one cycle of an alternating electric field is called the dielectric loss tangent, and it is represented by tan δ. The dielectric loss is proportional to the product of the dielectric constant ε and the dielectric loss tangent of the material. Thus, a total energy loss due to tan δ is increased in the higher frequency range. Also, since the heat release per unit area is larger when mounting high-density electronic elements, a material with a small tan δ must be used to suppress dielectric loss of the insulating material. By using a low dielectric polymer material with low dielectric loss, the heat release due to dielectric loss and electrical resistance is controlled, thus helping to reduce signal malfunction; materials with low transmission loss (energy loss) are therefore in great demand in the field of high frequency communications.
Various materials having such electrical properties including electrical resistance characteristics and low dielectric constant have been proposed, among which are thermoplastic resins such as polyolefins, vinyl chloride resins and fluorine-based resins, and thermosetting resins such as unsaturated polyester resins, polyimide resins, epoxy resins, vinyltriazine resins (VT resins), crosslinked polyphenylene oxide, curable polyphenylene ethers, and the like.
Polymers containing fluorine atoms in the molecular chain, such as vinylidene fluoride resins, trifluoroethylene resins and perfluoroethylene resins, have excellent electrical characteristics (low dielectric constant, low dielectric loss), heat resistance and chemical stability, but unlike thermoplastic resins they are poorly suited for shape forming and coat forming to obtain molds or films through heat treatment, and they increase costs when used for device manufacture. In addition, their low transparency is a drawback that limits the range of applicable fields. Since the above-mentioned low dielectric constant general use polymer materials all have allowable maximum temperatures of below 130° C., as electrical instrument insulating materials their heat resistance classification is type B or below as specified by JIS-C4003, and therefore their heat resistance is inadequate.
As resins with relatively good heat resistance there may be mentioned thermosetting resins such as epoxy resins, polyphenylene ether (PPE), unsaturated polyester resins and phenol resins. However, none of these exhibit a satisfactory level of heat resistance and dielectric constant.
Further performance demanded for low dielectric constant materials with excellent dielectric/insulating resistance characteristics often includes a requirement for soldering resistance that can withstand heating at 260° C. or above for at least 120 seconds because of the soldering step in the device manufacturing process.